1. Field of the Invention
The present invention relates to a positive or negative resist composition used in a resist pattern formation method that comprises an immersion lithography (immersion exposure) step, and a method of forming a resist pattern that uses such a resist composition.
2. Description of Related Art
Lithography techniques are widely used in the production of microscopic structures in a variety of electronic devices such as semiconductor devices and liquid crystal devices, and ongoing miniaturization of these device structures has lead to demands for further miniaturization of the resist patterns used in these lithography processes.
With current lithography techniques, using the most up-to-date ArF excimer lasers, fine resist patterns with a line width of approximately 90 nm are able to be formed, but in the future even finer pattern formation will be required.
In order to enable the formation of ultra fine patterns of less than 90 nm, the development of appropriate exposure apparatus and corresponding resists is the first requirement. In terms of exposure apparatus, apparatus with shorter wavelength light sources such as F2 lasers, EUV (extreme ultraviolet), electron beams and X-rays, or apparatus with larger lens numerical apertures (NA) are common.
However, reducing the wavelength of the light source requires a new and expensive exposure apparatus, and if the NA value is increased, then because the resolution and the depth of focus range exist in a trade-off type relationship, even if the resolution is increased, the depth of focus range reduces.
Against this background, a method known as immersion lithography has been reported (for example, see the non-patent reference 1, the non-patent reference 2, and the non-patent reference 3). This is a method in which, during exposure, the region between the lens and the resist layer disposed on top of the wafer, which has conventionally been filled with air or an inert gas such as nitrogen, is filled with a solvent such as pure water or a fluorine based inert liquid, which has a larger refractive index than the refractive index of air. By filling this region with this type of solvent, it is claimed that higher resolutions equivalent to those obtained using a shorter wavelength light source or a larger NA lens can be obtained using the same exposure light source wavelength, with no reduction in the depth of focus range.
Using this type of immersion lithography, resist patterns with higher resolution and a superior depth of focus can be formed at low cost, using lenses mounted in conventional apparatus, and consequently the method is attracting considerable attention.    (Non-Patent Reference 1) Journal of Vacuum Science and Technology B (U.S.), 1999, vol. 17, issue 6, pp. 3306-3309.    (Non-Patent Reference 2) Journal of Vacuum Science and Technology B (U.S.), 2001, vol. 19, issue 6, pp. 2353-2356.    (Non-Patent Reference 3) Proceedings of SPIE (U.S.), 2002, vol. 4691, pp. 459-465.
As described above, an advantage of immersion lithography is that in the production of semiconductor elements, which requires large investments in equipment and facilities, it is expected that large cost benefits and lithographical benefits such as improved resolution should be able to be realized in the semiconductor industry. However, because the resist layer comes in contact with a solvent during exposure as described above, a variety of problems can arise, such as degeneration of the resist layer, or the leaching of components from the resist that have a deleterious effect on the solvent, thus altering the refractive index of the solvent and impairing the inherent advantages offered by the immersion lithography process. Accordingly, it remains unclear whether or not immersion lithography is capable of forming a resist pattern of equivalent quality to those produced by conventional exposure processes. When an attempt was made to apply conventional KrF resists and ArF resist compositions to immersion lithography, a variety of problems emerged due to the effects of the solvent, including deterioration in the sensitivity, roughening of the surface of the resist pattern (a deterioration in the profile shape) such as the formation of T-top shaped resist patterns, or swelling of the resist pattern.